In order to efficiently transmit or record video images, there are used a video image encoding device (image encoding device) that generates encoded data by encoding video images, and a video image decoding device (image decoding device) that generates decoded images by decoding such encoded data. Specific video image coding schemes include, for example, the scheme implemented in the KTA software, which is a jointly developed codec by H.264/MPEG-4 AVC (NPL 1) and the Video Coding Experts Group (VCEG), the scheme implemented in the Test Model under Consideration (TMuC) software, as well as the scheme implemented in the successor codec, the Working Draft 1 of High-Efficiency Video Coding (NPL 2, hereinafter also called HEVC WD1).
In such coding schemes, an image (picture) constituting a video image is managed with a hierarchical structure made up of slices obtained by splitting an image, coding units obtained by splitting slices (also called macroblocks or CUs (coding units) in some cases), as well as blocks and partitions obtained by splitting coding units. Ordinarily, an image is encoded on a per-block basis.
Also, in such coding schemes, ordinarily a predicted image is generated on the basis of a locally decoded image obtained by encoding/decoding an input image, and the prediction residual (also called the “differential image” or “residual image”) obtained by subtracting the predicted image from the input image (original image) is encoded. Also, inter-frame prediction (inter prediction) and intra-frame prediction (intra prediction) may be cited as methods of generating predicted images.
With inter prediction, predicted images in a frame being decoded are generated in units of prediction units, by applying motion compensation using motion vectors, and by taking already-decoded frames as reference frames.
On the other hand, with intra prediction, predicted images in a frame being decoded are generated in units of prediction units, on the basis of already-decoded areas of the frame being decoded. One example of the intra prediction used in H.264/MPEG-4 AVC is a method that, for each prediction unit (each partition, for example), generates pixel values in that prediction unit by (1) selecting a prediction mode from a predetermined prediction mode group, and (2) extrapolating pixel values in an already-decoded area, in an extrapolation direction (prediction direct) that corresponds to the selected prediction mode (this method is also called “base prediction”).
Also, NPL 2 describes technology that reduces the bit rate of encoded data by skipping (Skip) or inferring (Merge) part of the information used for decoding. More specifically, from among the various information constituting encoded data used in the case of decoding a CU, information indicating a transform tree (TT) included in the CU and information indicating the splitting method for prediction units (PUs) in inter prediction are subjected to Skip. The transform tree (TT) includes information related to transform units (TUs) applied to each partial area within a CU. Meanwhile, information indicating the splitting method for prediction units (PUs) in inter prediction are subjected to Merge.
Additionally, from among the various information constituting encoded data used in the case of decoding the above prediction units (PUs), information indicating the types of inter prediction, information indicating reference images used in the case of inter prediction, and information indicating motion vectors are subjected to Skip or Merge.
In addition, NPL 3 discloses technology that changes the object of Merge from various information included in encoded data used in the case of decoding a CU to various information included in encoded data used in the case of decoding a PU.